1. Field of the Invention:
This invention relates to a process for producing .alpha.-type silicon nitride from chaff.
2. Discription of the Prior Art:
A lot of people have recently come to study the use of silicon nitride as a refractory material for the construction of a gas turbine, or the like. It is one of the ceramic materials which are expected to be widely used in the future.
Various methods have hitherto been proposed and studied for the synthesis of silicon nitride. They include:
(1) Nitriding silicon powder; PA0 (2) Vapor-phase reaction of silicon tetrachloride and ammonia, or silane and ammonia; and PA0 (3) Heating a mixture of silicon dioxide and carbon in a nitrogen atmosphere to reduce and nitride silicon.
The last method is particularly of great industrial value, since silicon dioxide is abundantly available. More specifically, there is known a method employing chaff which contains a relatively large quantity of silicon dioxide. According to this method, a carbide is prepared from chaff, and heated in the presence of nitrogen, whereby silicon dioxide in the carbide is reduced and nitrided to form .alpha.-type silicon nitride. This method has a number of advantages including the elimination of the step of mixing silicon dioxide and carbon, and the easy adjustment of the particle size of silicon nitride.
Chaff, however, contains a lot of elements, such as calcium, sodium, potassium, magnesium, manganese and aluminum, in addition to silicon. The silicon nitride obtained from chaff, therefore, contains those elements as impurities, and they lower the strength at a high temperature of a product made by sintering from such silicon nitride. These impurities are usually present in the form of oxides, including calcium oxide which is particularly undesirable as it greatly lowers the strength of the sintered product.